Bones and muscles of the legs and feet are adapted to absorb the forces generated during the walking and running gait cycle. These forces are referred to as shock. Particularly the foot has many adaptations for absorbing shock. The long arch and metatarsal arch of the foot play an important role in absorbing shock but the motion of the heel is extremely important. Motion of the heel not only absorbs shock but it plays an important role in locking and unlocking other joints which absorb much of the force of walking and running and which influence the ability to propel oneself in walking and running.
In order for the heel to move properly, also called pronation, during a stride the heel should strike the ground on the outside portion of the heel and it should rotate approximately 4 degrees inwardly during the time the foot is in contact with the ground. Rotation should be such that the ankle moves inward or toward the other ankle. Such rotation, as mentioned above, absorbs shock and adjusts the attitude of other joints and muscles to absorb shock.
A common disorder is for the legs and feet to be oriented so that the back of the heel or even the inside of the heel first strikes the ground during a stride so that rotation or pronation does not occur, or occurs to too small a degree, or occurs in a direction that does not perform the functions of orienting the rest of the skeleton and musculature to absorb shock.
This condition is normally treated by providing patients with devices called orthotics. Orthotics are usually in the form of thermo-plastic devices which fit into the shoes. The upper surface of an orthotic is made to conform to the shape of the appropriate portions of the patient's foot, usually by being heated to a plastic condition and solidified while held against a cast of the patient's foot. The orthotic normally has a shallow heel cup and extends forward to about the metatarsal arch.
After the upper surface of the orthotic is formed a plastic base or post is formed beneath the heel cup from a suitable material such as polymethylmethacrylate, after which the bottom of the heel-like post is ground to have two intersecting planes. The planes should intersect on a line between a point approximately 5 millimeters inwardly from the rearmost extension of the heel cup to a point tangent with the outermost and forwardmost portion of the orthotic. The planes should intersect at an angle of 4 degrees for the usual case although other angles may be employed for persons having greater or lesser needs for such correction.
Customarily, orthotics are prepared by obtaining a plaster cast of the patient's foot to obtain an impression of the contours of the patient's foot and then sending that contoured cast to a laboratory where a thermoplastic sheet is molded to the cast. The rear post portion is then attached and ground to provide the proper intersecting planes. Preparing the rear post requires expensive and highly skilled handwork to grind two intersecting planes at the correct angle with respect to each other and to have those planes intersect on a line between a particular point at the heel of the orthotic and a tangent to the edge. In addition, the preparation of orthotics by laboratory technicians causes a delay in the patient obtaining the orthotic due to the transportation time between the podiatrist's office and the laboratory and, of course, the time required to actually do the work.